Rapid spectroscopic determination of total water content



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May 17, 1955 H. P. BRolDA ETAL RAPID SPECTROSCOPIC DETERMINATION OFTOTAL WATER CONTENT Filed Oct. 7, 1953 wks.

INVENTOR Herber'zL P Broz'da Harold ./Morom'zz AGENT United StatesPatent O RAPID SPECTROSCOPIC DETERMINATION OF TOTAL WATER CONTENTHerbert P. Broida, Bethesda, and Harold J. Morowitz, Kensington, Md.

Application October 7, 1953, Serial No. 384,802

7 Claims. (Cl. 88-14) (Granted under Title 35, U. S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States for governmental purposeswithout the payment to me of any royalty thereon, in accordance with theprovisions of 35 United States Code (1952), section 266.

This invention concerns the problem of determining the total watercontent-water present in free, absorbed, hydrated, and/or bound form-ofvarious materials. This is a problem of great commercial and practicalimportance. For instance, millions of bushels of corn and other cerealproducts are sold annually on terms that take moisture content intoconsideration. The textile and cement industries, among others, alsohave important problems of water content determination. But until nowthere has been no easy and rapid way of determining total water content.The present invention provides a quick, simple method of determiningwith high precision the water content of practically any substance. Aknown amount of the substance is immersed in a mixture of hydrogen anddeuterium oxides; rapid spectroscopic determination of the ratio ofthese two oxides in the solution gives a measure of the Water content ofthe substance.

The classic general method of determining water content is by weightloss. A sample is heated to a room temperature high enough to drive 01Tthe water and is kept there until itsweight levels off. But when thismethod is used to determine the water content of corn, for instance, ahundred hours or more may be required for a determination. Even then,the accuracy of the determination leaves much to be desired.

More recently, some determinations of Water content have been made bymixing the sample with heavy water and analyzing the resulting solutionfor deuterium-hydrogen ratio by means of a mass spectrometer. Althoughthis method gives higher precision than the weight-loss method, it isstill very slow and requires more elaborate equipment than the method ofthe present invention.

Accuracy to within 1/10 of one percent of the true water content can beobtained by the method of the present invention. This accuracy isobtained even when the water content is small. For example, if the watercontent is only one percent of the total weight of the substance, thedetermination will be accurate to one part in 100,000 of the totalweight of the substance. Such a determination can be made in a totaltime of one hour or less. If this high a degree of accuracy is notrequired, a determination of 3 percent accuracy can be made in about 7minutes.

The irst object of the present invention is to provide a rapid andaccurate method for the determination of total water content ofsubstances.

Another object is to provide a new, more convenient, more practicalmethod for water-content determinations required in industry andcommerce.

Another object is to provide new, more convenient, methods for tracerstudies, using stable isotopes instead of radioactive isotopes.

Other uses and advantages of the invention will become 2,708,387Patented May 17, 1955 apparent upon reference to the specification anddrawing.

The figure shows a preferred form of equipment for use in Water contentdeterminations by the method of the invention.

To make a Water content determination, a sample of the material to beinvestigated is mixed with a solvent consisting of deuterium oxide or,more conveniently, with a measured volume ofdeuterium-oxide-hydrogenoxide mixture containing about 99 percent ofdeuterium oxide. The resulting solution contains water (hydrogen oxide)from the sample, and by comparing the D20 to H2O ratio of this resultingsolution with the D20 to H2O ratio of the original solvent the watercontent of the sample can be readily calculated. v

If the molar ratio of the hydrogen oxide to `deuterium oxide in theoriginal solvent is r, and the iinal molar ratio of the solution is R,then the fraction, f by weight ofthe sample which is water is given to arst approximation by the relation l where V is the volume of the solventin milliliters and M is the weight of the original sample in grams. Theapproximation, which is good to about 4 parts in 1000, is introduced byassuming that the partial molar volumes of hydrogen oxide and deuteriumoxide are the same, and that the density of water in the range of roomtemperature is 1,000.

The values of R and r are obtained by means of the apparatus shown inthe figure. Briefly, what this apparatus does is to ow vapor from adeuterium-oxidehydrogen-oxide mixture through an electrodeless dischargetube at reduced pressure. Light emitted by the vapor in this tube, whenthe vapor is excited by radio-frequency energy, is analyzed by means ofa recording spectrometer.

Referring to the drawing, the deuterium-oxide-hydrogen-oxide mixture 1to be analyzed is placed in a ask 2 which can be quickly connected to,or removed from, the rest of the system by means of connector 3. Air isremoved by means of a suitable vacuum pump 4. Vapor from ask 2 flowsthrough capillary constriction 6, ow gage 15, electrodeless dischargetube 8, and capillary constriction 7, where it is exhausted by vacuumpump 5. Constrictions 6 and 7 serve to control the flow of vapor throughtube 8 as Well as the pressure in tube 8. Pressure gage 10 indicates thepressure in tube 8. Fixed pressures in the range of 0.3 and 1.0 mm. ofmercury have been found satisfactory. The vapor in'tube 8 is excited byplacing tube 8 in a field of radio-frequency energy generated by anR.-F. exciter 9. Tube 8 is provided with a water jacket 11 for cooling.

It is important for accuracy that the vapor in the discharge tube ilowcontinuously during the analysis, so that only fresh vaporis analyzedatall times. If the vapor in the tube is closed oi, the percentage ofdeuterium tends to change with time.

The analysis depends on the wave-length separation of the emission linesof hydrogen and deuterium due to the isotopic shift. The ratio ofhydrogen to deuterium can be determined by measuring the ratio of the Hgline (4861.3 A.) to the D, line (4860.0 A.) coming from the discharge.Vapor in the discharge dissociates into H and OH and D and OD, and theobserved ratio of the intensity of the hydrogen to the deuterium linewill be a measure of the ratio of hydrogen oxide to deuterium oxide. Ithas been established that the ratio of excited hydrogen to deuterium isproportional to the ratio of the concentration of hydrogen oxide vaporto deuterium oxide vapor.

Still referring to the drawing, light from tube 8 passes throughcondensing lens 12 and entrance slit 13A to spherical mirror 14. Mirror14 reflects the light to diffraction grating 16; a grating ruled 12,000lines per centimeter has given satisfactory results. Grating 16 isslowly rotated by motor 17, and suitable speed-rcduction gearing ifneeded. Monochromatic light from grating 16 is reflected by mirror 14 toexit slit 1S. Photomultiplier tube 19 responds to the monochromaticlight passing through exit slit 18, The electrical output ofphotomultiplier tube 19 is amplified by directcurrent amplifier 21 andfed to strip-chart recorder 22.

With this arrangement, by rotating grating 16 to scan the desiredportion of the spectrum, a graphic recording showing the amplitudes ofthe Hp and D lines can be obtained in about 20 seconds.

In the table below are given experimental results obtained with themethod of the present invention when analyzing three different samples.It can be seen from this table that the experimental and theoreticalresults are in close agreement. Therefore the present invention providesa highly accurate as Well as a very rapid method for determining thetotal water content of bodies.

deuterium oxide and spectroanalyzing the resulting solution to determinethe relative proportions of deuterium oxide and hydrogen oxide presentin said solution.

2. The invention according to claim 1 in which said solvent comprises amixture of deuterium oxide and hydrogen oxide. Y

3. The invention according to claim 1 in which said solvent comprises amixture of deuterium oxide and hydrogen oxide in known proportions, atleast 50 percent by weight of said mixture being deuterium oxide.

4. The invention according to claim l in which said spectro-analyzingcomprises subjecting vapor from said solution, at reduced pressure, toradio-frequency excitation, said vapor being flowed continuously throughthe region in which it is subjected to said radio-frequency excitation;resolving the light emitted by said vapor when thus excited intomonochromatic light; and measuring photoelectrically the intensities ofat least one spectral line indicative of the presence of deuterium andat least one spectral line indicative of the presence of hydrogen.

5. The invention according to claim 4, said reduced Experimentaldetermination of total hydration Experimental Solvent Solutlon HydrationHydrate Solvent, Ratio Hy- Ratlo Hy- Material gram m1. drogen to drogento Egg stu' 'lhmlcal (M) (V) Deuterium Deuterium ar f ls/Ivla lon y raon (r) 11) ea CLISO4.5H2O 0. 770 1. O0 0. 011 0. 286 0. 354;l;0A 003 0.361 NazSzOLSHnO. 0. 648 1. 00 011 0. 249 0. 34zh0. O03 0. 363NEQSQO3.5H2O 0. 779 1. 00 0. 011 0. 289 0. 353:l:0. 003 0. 363

The method of the invention does not depend on 0bservation of the HB andD@ spectral lines mentioned above. Any pair of spectral lines can beobserved,

provided that one of them is characteristic of deuterium and the otherof hydrogen. The lines observed may be characteristic of molecules,including D20 and H2O, as well as of atoms. Other spectro-analytic meansthan those described can be used.

What is claimed is:

1. The method of determining the total water content of a substance thesteps of which comprise mixing a sample of the substance with a solventcontaining No references cited.

1. THE METHOD OF DETERMINING THE TOTAL WATER CONTENT OF A SUBSTANCE THESTEPS OF WHICH COMPRISE MIXING A SAMPLE OF THE SUBSTANCE WITH A SOLVENTCONTAINING DEUTERIUM OXIDE AND SPECTROANALYZING THE RESULTING SO-